This invention relates generally to the field of captive type spark plug gaskets, and more particularly to an improved type offering several significant advantages over the prior art.
In my prior U.S. Pat. No. 4,064,612, there is disclosed a captive type spark plug gasket and a tool for staking the gasket onto the threaded shank of a spark plug body. This construction is particularly suited to that type of plug in which the shank is threaded throughout its entire length, without the provision of an undercut at the base thereof where it joins the main portion of the plug body. This patent teaches the necessity of staking the gasket in such manner that the locking lugs are deformed against the threads, and the portions of the gasket performing the sealing functions are not disturbed. In the case of spark plug bodies having the undercut portion on the shank, the industry has used the type of gasket disclosed in my prior U.S. Pat. No. 3,299,504. In this type of gasket, the locking flange which is deformed into the undercut area projects longitudinally outwardly of the S-folded sealing element, resulting in a gasket of substantially greater longitudinally height, and also resulting in an S-fold construction in which the interstices between the sealing members are so narrow that when the gasket is immersed in a synthetic resinous coating material, the material does not flow readily into the interstice by capillary action, and thus with compression of the S-fold on installation, and subsequent distortion, a potential uncoated area is exposed.
With resort to mechnization for installing the gaskets on the plug bodies, the desirability of lowering the profile of the gaskets to facilitate orientation by hoppers feeding chutes which supply the gaskets to upturned plug bodies becomes apparent. Thus, the configuration shown in my U.S. Pat. No. 3,299,504 has become undesirable. The configuration disclosed in my U.S. Pat. No. 4,064,612 is preferable, but in the disclosed form, it is not suitable for use with an undercut shank, because the degree of radially inward movement of the locking lugs under the action of the staking tool is far less than adequate. In the case of a fully threaded shank, the requirement is relatively small with respect to inward movement, and high with respect to required mechanical force, owing to the necessity of at least partially conforming the locking tab to the threaded surface of the shank.